This invention relates to an ignition current conduction time control apparatus for an internal combustion engine.
Various techniques for separately controlling current applied to the ignition coils of the individual cylinders of an internal combustion engine are disclosed in JP-A-58-197470 and Japanese Utility Model Application Laid-Open No. 62-135869. JP-A-58-197470 proposes a technique for determining the crank angle when the cylinder pressure is maximum for each cylinder, and controlling ignition timing so that the crank angle values are the same for all cylinders. Japanese Utility Model Application Laid-Open No. 62-135869 discloses a technique for controlling the start and the end of current conduction to the primary side of the ignition coil for each cylinder according to the characteristics of the cylinders.
In both JP-A-58-197470 and Japanese Utility Model Application Laid-Open No. 62-135869, a current is applied to the ignition coil for the individual cylinders, but no consideration is given to the variation in the characteristics of the ignition coils for the individual cylinders.
In ignition control at high engine speed (more, than 6000 rpm, for example), it is necessary to cause the operational current applied to the ignition coils to rise sharply. This rise is determined by a circuit time constant (R/L). R denotes the resistance component of the internal resistance, of the coil, for example, and L denotes the inductance component of the coil. Therefore, in order to achieve a sharp rise in the current, it is necessary to provide a small R and a large L. However, this time constant (R/L) varies with the cylinders, and as a result, it is impossible to provide for a uniform rise of the current applied to the ignition coils for the individual cylinders.
The variation in the circuit time constants (R/L) chiefly results from the quality irregularity in the manufacture of the parts of the ignition coils. In addition, the circuit time constant sometimes varies as the resistance component R changes with a change in temperature. The reference voltage to the collector of the power transistor for control of the operating current to each ignition coil is supplied from a power source (a battery or a generator). If this source voltage changes, the I component in the energy LI.sup.2 accumulated in the ignition coil changes, causing the ignition energy to change, which results in an uncalled-for phenomenon.
The above-mentioned two prior-art techniques give no consideration to the control of current conduction time when there are variations in the inductance L and the resistance R of the ignition coils and when there are changes in the operating conditions, including the ambient temperature, the source voltage, etc., nor do the publications provide any description of a method of reducing the current conduction time when it becomes excessive. For example, a problem resulting from an excessive current conduction time is that when the current conduction time is long, the amount of generated heat of the primary coil increases, which is another uncalled-for phenomenon.